The U.S. Pat. No. 4,566,855 discloses a propeller consisting of a propelling unit having a plurality of helical blades serving to propel the craft by the backward thrust of water, a driving unit driven by the engine, and a rubber sleeve located between the driving unit and the propelling unit. The connecting portions of the propelling unit, the sleeve, and the driving unit are provided respectively with a plurality of elongate projections or retaining slots enabling the output torsion of the engine to cause the propelling unit to rotate via the driving unit and the sleeve, so as to generate the backward thrust of water to propel the boat.
The elastic sleeve is capable of absorbing partially the impact force generated instantly by the helical blades upon being impacted on by a foreign object present in water, thereby mitigating the destructive effect of the impact force on the blades and the engine drive shaft.
Such a prior art propeller as described above is defective in design in that the elastic sleeve is rather expensive, and that the elastic sleeve of a high quality is hard to come by, and further that the shock-absorbing effectiveness of the elastic sleeve depends greatly on the rigidity of the sleeve. For example, if the sleeve is insufficiently rigid, the propelling unit can not be effectively actuated to rotate. On the contrary, if the sleeve is excessively rigid, the shock-absorbing capability of the sleeve is greatly undermined.
Another U.S. Pat. No. 4,826,404 discloses a propeller consisting of a propelling unit and the driving unit, which are provided respectively with a plurality of rod-shaped elastic members capable of mitigating an impact force exerting on the blades and capable of being destroyed to cause the propelling unit and the driving unit to idle, so as to protect the propeller blades and the engine driving the propeller blades. However, the destruction of the elastic members can result in the incapacitating of the boat. Moreover, the job of replacing the destroyed elastic member can not be done by an amateur without a professional assistance.